The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with gradient coils for a magnetic resonance imaging apparatus and will be described with particular reference thereto. However, it is to be appreciated that the present invention will also find application in conjunction with localized magnetic resonance spectroscopy systems and other applications which utilize gradient magnetic fields.
In magnetic resonance imaging, a uniform magnetic field is created through an examination region in which a subject to be examined is disposed. A series of radio frequency pulses and magnetic field gradients are applied to the examination region to excite and manipulate magnetic resonance. Gradient fields are conventionally applied as a series of gradient pulses with pre-selected profiles. The radio frequency pulses excite magnetic resonance and the gradient field pulses phase and frequency encode the induced resonance. In this manner, phase and frequency encoded magnetic resonance signals are generated.
More specifically, the gradient magnetic field pulses are typically applied to select and encode the magnetic resonance with spatial position. In some embodiments, the magnetic field gradients are applied to select a slice or slab to be imaged. Ideally, the phase or frequency encoding uniquely identifies spatial location.
Conventionally, the uniform main magnetic field is generated in one of two ways. The first method employs a cylindrically shaped solenoidal main magnet. The central bore of the main magnet defines the examination region in which a horizontally directed main magnetic field is generated. The second method employs a main magnet having opposing poles arranged facing one another to define therebetween the examination region. The poles are typically connected by a C-shaped or a four post ferrous flux return path. This configuration generates a vertically directed main magnetic field within the examination region. The C-shaped main magnet, often referred to as having open magnet geometry, has resolved important MRI issues, such as increasing the patient aperture, avoiding patient claustrophobia, and improving access for interventional MRI applications. However, the design of gradient coils for generating linear magnetic field gradients differs from that for the bore-type horizontal field system due to the direction of the magnetic field.
When designing gradient coils for magnetic resonance imaging, many opposing factors must be considered. Typically, there is a trade off between gradient speed and image quality factors, such as volume, uniformity, and linearity. Some magnetic resonance sequences require a gradient coil which emphasizes efficiency, while other sequences are best with a gradient coil which emphasizes image quality factors. For example, a gradient coil which has a large linear imaging volume is advantageous for spine imaging, but is disadvantageous in terms of the dB/dt when switched with a high slew rate.
Open magnetic systems with vertically directed fields are attractive for MRI applications because an open magnet geometry increases the patient aperture and increases access for interventional MRI applications. In such open magnet systems, it has been known to use a bi-planar gradient coil assembly for generation of magnetic field gradients. However, the use of this type of bi-planar gradient coil assembly somewhat detracts from the purposes for using an open magnet geometry in that it reduces the patient aperture and diminishes access for interventional procedures by employing two planar gradient coils, one on either side of the subject being examined. In addition, the performance of the bi-planar configuration often suffers in terms of its gradient strength, slew rate, and dB/dt levels.
A single uniplanar gradient coil may remedy some of the aforementioned bi-planar shortcomings in regard to gradient strength and slew rate. However, such a structure often suffers from a reduced field of view, which affects applications requiring larger spatial coverage, such as spinal imaging. In order to increase the spatial coverage provided by a uniplanar or bi-planar gradient coil, the uniformity and linearity of the gradient magnetic field must be improved. In addition, the strength of the gradient field must be increased to address the demand for a higher resolution image. These two factors have a detrimental effect on the dB/dt level for the gradient coil.
The present invention contemplates a new and improved gradient coil assembly which overcomes the above-referenced problems and others.